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One of the biggest obstacles to developing better zeolite-based catalysts is the lack of methods for quantitatively locating light heteroatoms on the T-sites in zeolites. Titanium silicalite-1 (TS-1) is a Ti-bearing zeolite-type catalyst commonly used in partial oxidation reactions with H 2 O 2 , such as aromatic hydroxylation and olefin epoxidation. The reaction mechanism is controlled by the configuration of titanium sites replacing silicon in the zeolite framework, but these sites remain unknown, hindering a fundamental understanding of the reaction. This study quantitatively determines heteroatoms within the zeolite-type framework using anomalous X-ray powder diffraction (AXRD) and the changes in the titanium X-ray scattering factor near the Ti K-edge (4.96 keV). Two TS-1 samples, each with approximately 2 Ti atoms per unit cell, were examined. Half of the titanium atoms are primarily split between sites T3 and T9, with the remainder dispersed among various T-sites within both MFI-type frameworks. One structure showed significant non-framework titanium in the micropores of a more distorted lattice. In both samples, isolated titanium atoms were more prevalent than dinuclear species, which could only potentially arise at site T9, but with a significant energy penalty and were not detected., (© 2024. The Author(s).)

The catalytic pyrolysis of guaiacol-based lignin monomers, vanillin, syringol, and eugenol over commercial HZSM-5 has been investigated using operando Photoelectron Photoion Coincidence (PEPICO) spectroscopy to unveil the reaction mechanism by detecting reactive intermediates, such as quinone methides and ketenes, and products. Vanillin shares the decomposition mechanism with guaiacol due to prompt and efficient decarbonylation, which allows us to control this reaction leading to a phenol selectivity increase by switching to a faujasite catalyst and decreasing the Si/Al ratio. Syringol first demethylates to 3-methoxycatechol, which mainly dehydroxylates to o - and m -guaiacol. Ketene formation channels over HZSM-5 are less important here than for guaiacol or vanillin, but product distribution remains similar. C 3 addition to guaiacol yields eugenol , which shows a more complex product distribution upon catalytic pyrolysis. By analogies to monomers with simplified functionalization, namely allylbenzene, 4-allylcatechol, and 4-methylcatechol, the eugenol chemistry could be fully resolved. Previously postulated reactive semi-quinone intermediates are detected spectroscopically, and their involvement opens alternative pathways to condensation and phenol formation. Allyl groups, produced by dehydroxylation of the β-O-4 bond, may not only decompose via C1/C2/C3 loss, but also cyclize to indene and its derivatives over HZSM-5. This comparably high reactivity leads to an unselective branching of the chemistry and to a complex product distribution, which is difficult to control. Indenes and naphthalenes are also prototypical coke precursors efficiently deactivating the catalyst. We rely on these mechanistic insights to discuss strategies to fine-tune process conditions to increase the selectivities of desired products by enhancing either vanillin and guaiacol or supressing eugenol yields from native lignin., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Copper-exchanged zeolite omega (Cu-omega) is a potent material for the selective conversion of methane-to-methanol (MtM) via the oxygen looping approach. However, its performance exhibits substantial variation depending on the operational conditions. Under an isothermal temperature regime, Cu-omega demonstrates subdued activity below 230 °C, but experiences a remarkable increase in activity at 290 °C. Applying a high-temperature activation protocol at 450 °C causes a rapid deactivation of the material. This behavioral divergence is investigated by combining reactivity studies, neutron and in situ high-resolution anomalous X-ray powder diffraction (HR-AXRPD), as well as electron paramagnetic resonance spectroscopy, to reveal that the migration of Cu throughout the framework is the primary cause of these behaviors, which in turn is governed by the degree of hydration of the system. This work suggests that control over the Cu migration throughout the zeolite framework may be harnessed to significantly increase the activity of Cu-omega by generating more active sites for the MtM conversion. These results underscore the power of in situ HR-AXRPD for unraveling the behavior of materials under reaction conditions and suggest that a re-evaluation of Cu-zeolites priorly deemed inactive for the MtM conversion across a broader range of conditions and looping protocols may be warranted., (© 2024 Wiley‐VCH GmbH.)

The CH4 oxidation performance of Cu-chabazite zeolites characterized by distinct Si/Al ratios and Cu loadings has been studied and the observed variations in reactivity have been correlated to the differences in the nature of the formed active centers. Plug flow reactor tests, in situ Fourier-transform infrared, and X-ray absorption spectroscopy demonstrate that a decrease in Cu loading shifts the reactivity/redox profile to higher temperatures and increases the CH3OH selectivity and Cu-efficiency. In situ electron paramagnetic resonance, Raman, ultraviolet-visible, Fourier-transform infrared, and photoluminescence spectroscopies reveal that this behavior is associated with the presence of monomeric Cu active sites, including bare Cu2+ and [CuOH]+ present at low Si/Al ratio and Cu loading. Formation of two distinct [Cu2(µ-O)]2+ moieties at higher Si/Al ratio or Cu loading forces these trends into the opposite direction. Operando electron paramagnetic resonance and ultraviolet-visible spectroscopy show that the apparent activation energy of monomeric Cu active species decreases with increasing Si/Al ratio, whereas the one of dimeric centers is unaffected., (© 2024 Wiley‐VCH GmbH.)

The epoxidation of ethylene stands as one of the most important industrial catalytic reactions, and silver-based catalysts show superior activity and selectivity. Oxygen is activated on the surface of silver during the reaction and exerts a substantial impact on product selectivity. Notably, the oxygen species residing in the topmost atomic layers profoundly influence the reactivity of a catalyst. However, their characterization under in situ reaction conditions remains a huge challenge, and specific structures have not been identified yet. In this study, we employ in situ X-ray photoelectron spectroscopy and density functional theory calculations to determine the oxygen species formed at the topmost atomic layers of a silver foil and to assign them a structure. Three different groups of oxygen species activated on silver are identified: (i) surface lattice oxygen and two oxygen species originating from associatively adsorbed dioxygen and (ii) top and (iii) subsurface oxygen. Transient in situ photoelectron spectroscopy experiments are carried out to reveal the dynamic evolution and thus reactivity of the different oxygen species under ethylene epoxidation reaction environments. The top oxygen atom from the adsorbed associated dioxygen is the most active. Meanwhile, a frequency-selective data analysis method, developed to process time-resolved data, provides insights into the evolving trends of peak intensities for different oxygen species. The versatility of this method suggests its potential application in future time-resolved characterization studies., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

An interplay between Pd and PdO and their spatial distribution inside the particles are relevant for numerous catalytic reactions. Using in situ time-resolved X-ray absorption spectroscopy (XAS) supported by theoretical simulations, a mechanistic picture of the structural evolution of 2.3 nm palladium nanoparticles upon their exposure to molecular oxygen is provided. XAS analysis revealed the restructuring of the fcc-like palladium surface into the 4-coordinated structure of palladium oxide upon absorption of oxygen from the gas phase and formation of core@shell Pd@PdO structures. The reconstruction starts from the low-coordinated sites at the edges of palladium nanoparticles. Formation of the PdO shell does not affect the average Pd‒Pd coordination numbers, since the decrease of the size of the metallic core is compensated by a more spherical shape of the oxidized nanoparticles due to a weaker interaction with the support. The metallic core is preserved below 200 °C even after continuous exposure to oxygen, with its size decreasing insignificantly upon increasing the temperature, while above 200 °C, bulk oxidation proceeds. The Pd‒Pd distances in the metallic phase progressively decrease upon increasing the fraction of the Pd oxide due to the alignment of the cell parameters of the two phases., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

The activity and durability of the Cu/ZnO/Al 2 O 3 (CZA) catalyst formulation for methanol synthesis from CO/CO 2 /H 2 feeds far exceed the sum of its individual components. As such, this ternary catalytic system is a prime example of synergy in catalysis, one that has been employed for the large scale commercial production of methanol since its inception in the mid 1960s with precious little alteration to its original formulation. Methanol is a key building block of the chemical industry. It is also an attractive energy storage molecule, which can also be produced from CO 2 and H 2 alone, making efficient use of sequestered CO 2 . As such, this somewhat unusual catalyst formulation has an enormous role to play in the modern chemical industry and the world of global economics, to which the correspondingly voluminous and ongoing research, which began in the 1920s, attests. Yet, despite this commercial success, and while research aimed at understanding how this formulation functions has continued throughout the decades, a comprehensive and universally agreed upon understanding of how this material achieves what it does has yet to be realized. After nigh on a century of research into CZA catalysts, the purpose of this Review is to appraise what has been achieved to date, and to show how, and how far, the field has evolved. To do so, this Review evaluates the research regarding this catalyst formulation in a chronological order and critically assesses the validity and novelty of various hypotheses and claims that have been made over the years. Ultimately, the Review attempts to derive a holistic summary of what the current body of literature tells us about the fundamental sources of the synergies at work within the CZA catalyst and, from this, suggest ways in which the field may yet be further advanced.

The local reaction environment of catalytic active sites can be manipulated to modify the kinetics and thermodynamic properties of heterogeneous catalysis. Because of the unique physical-chemical nature of water, heterogeneously catalyzed reactions involving specific interactions between water molecules and active sites on catalysts exhibit distinct outcomes that are different from those performed in the absence of water. Zeolitic materials are being applied with the presence of water for heterogeneous catalytic reactions in the chemical industry and our transition to sustainable energy. Mechanistic investigation and in-depth understanding about the behaviors and the roles of water are essentially required for zeolite chemistry and catalysis. In this review, we focus on the discussions of the nature and structures of water adsorbed/stabilized on Brønsted and Lewis acidic zeolites based on experimental observations as well as theoretical calculation results. The unveiled functions of water structures in determining the catalytic efficacy of zeolite-catalyzed reactions have been overviewed and the strategies frequently developed for enhancing the stabilization of zeolite catalysts are highlighted. Recent advancement will contribute to the development of innovative catalytic reactions and the rationalization of catalytic performances in terms of activity, selectivity and stability with the presence of water vapor or in condensed aqueous phase.

ConspectusMethane complete oxidation is an important reaction that is part of the general scheme used for removing pollutants contained in emissions from internal combustion engines and, more generally, combustion processes. It has also recently attracted interest as an option for the removal of atmospheric methane in the context of negative emission technologies. Methane, a powerful greenhouse gas, can be converted to carbon dioxide and water via its complete oxidation. Despite burning methane being facile because the combustion sustains its complete oxidation after ignition, methane strong C-H bonds require a catalyst to perform the oxidation at low temperatures and in the absence of a flame so as to avoid the formation of nitrogen oxides, such as those produced in flares. This process allows methane removal to be obtained under conditions that usually lead to higher emissions, such as under cold start conditions in the case of internal combustion engines. Among several options that include homo- and heterogeneous catalysts, supported palladium-based catalysts are the most active heterogeneous systems for this reaction. Finely divided palladium can activate C-H bonds at temperatures as low as 150 °C, although complete conversion is usually not reached until 400-500 °C in practical applications. Major goals are to achieve catalytic methane oxidation at as low as possible temperature and to utilize this expensive metal more efficiently.Compared to any other transition metal, palladium and its oxides are orders of magnitude more reactive for methane oxidation in the absence of water. During the last few decades, much research has been devoted to unveiling the origin of the high activity of supported palladium catalysts, their active phase, the effect of support, promoters, and defects, and the effect of reaction conditions with the goal of further improving their reactivity. There is an overall agreement in trends, yet there are noticeable differences in some details of the catalytic performance of palladium, including the active phase under reaction conditions and the reasons for catalyst deactivation and poisoning. In this Account we summarize our work in this space using well-defined catalysts, especially model palladium surfaces and those prepared using colloidal nanocrystals as precursors, and spectroscopic tools to unveil important details about the chemistry of supported palladium catalysts. We describe advanced techniques aimed at elucidating the role of several parameters in the performance of palladium catalysts for methane oxidation as well as in engineering catalysts through advancing fundamental understanding and synthesis methods. We report the state of research on active phases and sites, then move to the role of supports and promoters, and finally discuss stability in catalytic performance and the role of water in the palladium active phase. Overall, we want to emphasize the importance of a fundamental understanding in designing and realizing active and stable palladium-based catalysts for methane oxidation as an example for a variety of energy and environmental applications of nanomaterials in catalysis.

Zirconium-containing metal-organic framework (MOF) with UiO-66 topology is an extremely versatile material, which finds applications beyond gas separation and catalysis. However, after more than 10 years after the first reports introducing this MOF, understanding of the molecular-level mechanism of its nucleation and growth is still lacking. By means of in situ time-resolved high-resolution mass spectrometry, Zr K-edge X-ray absorption spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, and X-ray diffraction it is showed that the nucleation of UiO-66 occurs via a solution-mediated hydrolysis of zirconium chloroterephthalates, whose formation appears to be autocatalytic. Zirconium-oxo nodes form directly and rapidly during the synthesis, the formation of pre-formed clusters and stable non-stoichiometric intermediates are not observed. The nuclei of UiO-66 possess identical to the crystals local environment, however, they lack long-range order, which is gained during the crystallization. Crystal growth is the rate-determining step, while fast nucleation controls the formation of the small crystals of UiO-66 with a narrow size distribution of about 200 nanometers., (© 2023 The Authors. Small published by Wiley-VCH GmbH.)

Copper(II)-containing mordenite (CuMOR) is capable of activation of C-H bonds in C 1 -C 3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with Cu I sites, formed upon reduction of Cu II -oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

In this work, we investigated cyclohexane oxidative dehydrogenation (ODH) catalyzed by cobalt ferrite nanoparticles supported on reduced graphene oxide (RGO). We aim to identify the active sites that are specifically responsible for full and partial dehydrogenation using advanced spectroscopic techniques such as X-ray photoelectron emission microscopy (XPEEM) and X-ray photoelectron spectroscopy (XPS) along with kinetic analysis. Spectroscopically, we propose that Fe 3+ /T d sites could exclusively produce benzene through full cyclohexane dehydrogenation, while kinetic analysis shows that oxygen-derived species (O*) are responsible for partial dehydrogenation to form cyclohexene in a single catalytic sojourn. We unravel the dynamic cooperativity between octahedral and tetrahedral sites and the unique role of the support in masking undesired active (Fe 3+ /T d ) sites. This phenomenon was strategically used to control the abundance of these species on the catalyst surface by varying the particle size and the wt % content of the nanoparticles on the RGO support in order to control the reaction selectivity without compromising reaction rates which are otherwise extremely challenging due to the much favorable thermodynamics for complete dehydrogenation and complete combustion under oxidative conditions., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The methane-to-methanol (MtM) conversion via the oxygen looping approach using copper-exchanged zeolites has been extensively studied over the last decade. While a lot of research has focussed on maximizing yield and selectivity, little has been directed toward productivity-a metric far more meaningful for evaluating industrial potential. Using copper-exchanged zeolite omega (Cu-omega), a material highly active and selective for the MtM conversion using the isothermal oxygen looping approach, we show that this material exhibits unprecedented potential for industrial valorization. In doing so, we also present a novel methodology combining operando XAS and mass spectrometry for the screening of materials for the MtM conversion in oxygen looping mode., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Cu-exchanged mordenite (MOR) is a promising material for partial CH 4 oxidation. The structural diversity of Cu species within MOR makes it difficult to identify the active Cu sites and to determine their redox and kinetic properties. In this study, the Cu speciation in Cu-MOR materials with different Cu loadings has been determined using operando electron paramagnetic resonance (EPR) and operando ultraviolet-visible (UV/Vis) spectroscopy as well as in situ photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy. A novel pathway for CH 4 oxidation involving paired [CuOH] + and bare Cu 2+ species has been identified. The reduction of bare Cu 2+ ions facilitated by adjacent [CuOH] + demonstrates that the frequently reported assumption of redox-inert Cu 2+ centers does not generally apply. The measured site-specific reaction kinetics show that dimeric Cu species exhibit a faster reaction rate and a higher apparent activation energy than monomeric Cu 2+ active sites highlighting their difference in the CH 4 oxidation potential., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Even though confinement was identified as a common element of selective catalysis and simulations predicted enhanced properties of adsorbates within microporous materials, experimental results on the characterization of the adsorbed phase are still rare. In this study, we provide experimental evidence of the increase of propene density in the channels of Zn-MOF-74 by 16(2)% compared to the liquid phase. The ordered propene molecules adsorbed within the pores of the MOF have been localized by in situ neutron powder diffraction, and the results are supported by adsorption studies. The formation of a second adsorbate layer, paired with nanoconfinement-induced short intermolecular distances, causes the efficient packing of the propene molecules and results in an increase of olefin density., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

While the structures of Brønsted acid sites (BAS) in zeolites are well understood, those of Lewis acid sites (LAS) remain an active area of investigation. Under hydrated conditions, the reversible formation of framework-associated octahedral aluminum has been observed in zeolites in the acidic form. However, the structure and formation mechanisms are currently unknown. In this work, combined experimental 27 Al NMR spectroscopy and computational data reveal for the first time the details of the zeolite framework-associated octahedral aluminium. The octahedral LAS site becomes kinetically allowed and thermodynamically stable under wet conditions in the presence of multiple nearby BAS sites. The critical condition for the existence of such octahedral LAS appears to be the availability of three protons: at lower proton concentration, either by increasing the Si/Al or by ion-exchange to non-acidic form, the tetrahedral BAS becomes thermodynamically more stable. This work resolves the question about the nature and reversibility of framework-associated octahedral aluminium in zeolites., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Unveiling catalytic mechanisms at a molecular level aids rational catalyst design and selectivity control for process optimization. In this study, we find that the Brønsted acid site density of the zeolite catalyst efficiently controls the guaiacol catalytic pyrolysis mechanism. Guaiacol demethylation to catechol initiates the reaction, as evidenced by the detected methyl radicals. The mechanism branches to form either fulvenone (c-C 5 H 4  = C = O), a reactive ketene intermediate, by catechol dehydration, or phenol by acid-catalyzed dehydroxylation. At high Brønsted acid site density, fulvenone formation is inhibited due to surface coordination configuration of its precursor, catechol. By quantifying reactive intermediates and products utilizing operando photoelectron photoion coincidence spectroscopy, we find evidence that ketene suppression is responsible for the fivefold phenol selectivity increase. Complementary fulvenone reaction pathway calculations, along with 29 Si NMR-MAS spectroscopy results corroborate the mechanism. The proposed, flexible operando approach is applicable to a broad variety of heterogeneous catalytic reactions., (© 2023. The Author(s).)

The tandem hydroformylation-aldol condensation (tandem HF-AC) reaction offers an efficient synthetic route to the synthesis of industrially relevant products. The addition of Zn-MOF-74 to the cobalt-catalyzed hydroformylation of 1-hexene enables tandem HF-AC under milder pressure and temperature conditions than the aldox process, where zinc salts are added to cobalt-catalyzed hydroformylation reactions to promote aldol condensation. The yield of the aldol condensation products increases by up to 17 times compared to that of the homogeneous reaction without MOF and up to 5 times compared to the aldox catalytic system. Both Co 2 (CO) 8 and Zn-MOF-74 are required to significantly enhance the activity of the catalytic system. Density functional theory simulations and Fourier-transform infrared experiments show that heptanal, the product of hydroformylation, adsorbs on the open metal site (OMS) of Zn-MOF-74, thereby increasing the electrophilic character of the carbonyl carbon atom and facilitating the condensation., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Platinum nanoparticles (NPs) supported by titania exhibit a strong metal-support interaction (SMSI) [1] that can induce overlayer formation and encapsulation of the NP's with a thin layer of support material. This encapsulation modifies the catalyst's properties, such as increasing its chemoselectivity [2] and stabilizing it against sintering. [3] Encapsulation is typically induced during high-temperature reductive activation and can be reversed through oxidative treatments. [1] However, recent findings indicate that the overlayer can be stable in oxygen. [4, 5] Using in situ transmission electron microscopy, we investigated how the overlayer changes with varying conditions. We found that exposure to oxygen below 400 °C caused disorder and removal of the overlayer upon subsequent hydrogen treatment. In contrast, elevating the temperature to 900 °C while maintaining the oxygen atmosphere preserved the overlayer, preventing platinum evaporation when exposed to oxygen. Our findings demonstrate how different treatments can influence the stability of nanoparticles with or without titania overlayers. expanding the concept of SMSI and enabling noble metal catalysts to operate in harsh environments without evaporation associated losses during burn-off cycling., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful tool to characterize the surface structure of heterogeneous catalysts in situ . In order to improve the time resolution and the signal-to-noise (S/N) ratio of photoemission spectra, we collected consecutive APXP spectra during the periodic perturbation of a powder Pd/Al 2 O 3 catalyst away from its equilibrium state according to the modulated excitation approach (ME). Averaging of the spectra along the alternate pulses of O 2 and CO improved the S/N ratio demonstrating that the time resolution of the measurement can be limited solely to the acquisition time of one spectrum. Through phase sensitive analysis of the averaged time-resolved spectra, the formation/consumption dynamics of three oxidic species, two metal species, adsorbed CO on Pd 0 as well as Pd n + ( n > 2) was followed along the gas switches. Pd n + and 2-fold surface PdO species were recognised as most reactive to the gas switches. Our approach demonstrates that phase sensitive detection of time-resolved XPS data allows following the dynamics of reactive species at the solid-gas interface under different reaction environments with unprecedented precision., Competing Interests: The authors have no competing interests to declare that are relevant to the content of this article., (This journal is © The Royal Society of Chemistry.)

Understanding the reaction mechanism is critical yet challenging in heterogeneous catalysis. Reactive intermediates, e.g., radicals and ketenes, are short-lived and often evade detection. In this review, we summarize recent developments with operando photoelectron photoion coincidence (PEPICO) spectroscopy as a versatile tool capable of detecting elusive intermediates. PEPICO combines the advantages of mass spectrometry and the isomer-selectivity of threshold photoelectron spectroscopy. Recent applications of PEPICO in understanding catalyst synthesis and catalytic reaction mechanisms involving gaseous and surface-confined radical and ketene chemistry will be summarized., (Copyright 2023 Z. Zhang, J. Pérez-Ramírez, J. A. van Bokhoven, A. Bodi, P. Hemberger. License: This work is licensed under a Creative Commons Attribution 4.0 International License.)

Operando X-ray absorption spectroscopy identified that the concentration of Fe 2+ species in the working state-of-the-art Pt-FeO x catalysts quantitatively correlates to their preferential carbon monoxide oxidation steady-state reaction rate at ambient temperature. Deactivation of such catalysts with time on stream originates from irreversible oxidation of active Fe 2+ sites. The active Fe 2+ species are presumably Fe +2 O -2 clusters in contact with platinum nanoparticles; they coexist with spectator trivalent oxidic iron (Fe 3+ ) and metallic iron (Fe 0 ) partially alloyed with platinum. The concentration of active sites and, therefore, the catalyst activity strongly depends on the pretreatment conditions. Fe 2+ is the resting state of the active sites in the preferential carbon monoxide oxidation cycle., (© 2022 Wiley-VCH GmbH.)

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum-ceria catalyst. At low temperatures (25-100 °C and 1 mbar H 2 ) surface O-H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce 4+ to Ce 3+ . This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H 2 ), O-H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring.

Preparation conditions have a vital effect on the structure of alumina-supported hydrodesulfurization (HDS) catalysts. To explore this effect, we prepared two NiMoS/Al 2 O 3 catalyst samples with the same target composition using different chemical sources and characterizing the oxidic NiMo precursors and sulfided and spent catalysts to understand the influence of catalyst structure on performance. The sample prepared from ammonium heptamolybdate and nickel nitrate (sample A) contains Mo in the oxidic precursor predominantly in tetrahedral coordination in the form of crystalline domains, which show low reducibility and strong metal-support interactions. This property influences the sulfidation process such that the sulfidation processes of Ni and Mo occur tendentially separately with a decreased efficiency to form active Ni-Mo-S particles. Moreover, inactive unsupported MoS 2 particles or isolated NiS x species are formed, which are either washed off during catalytic reaction or aggregated to larger particles as seen in scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDX). The oxidic precursor of the sample synthesized using nickel carbonate and molybdenum trioxide as metal sources (sample B), however, contains Mo in octahedral coordination and shows higher reducibility of the metal species as well as weaker metal-support interactions than that of sample A; these properties allow an efficient sulfidation of Mo and Ni such that formation of active Ni-Mo-S particles is the main product. Ptychographic X-ray computed tomography (PXCT) and STEM and EDX measurements show that the structure formed during sulfidation is stable under operation conditions. The structural differences explain the HDS activity difference between these two samples and explain why sample B is much active than sample A., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Oxides are essential catalysts and supports for noble metal catalysts. Their interaction with hydrogen enables, e.g., their use as a hydrogenation catalyst. Among the oxides considered reducible, substantial differences exist in their capability to activate hydrogen and how the oxide structure transforms due to this interaction. Noble metals, like platinum, generally enhance the oxide reduction by hydrogen spillover. This work presents a systematic temperature-programmed reduction study (300 to 873 K) of iron oxide, ceria, titania, zirconia, and alumina, with and without supported platinum. For all catalysts, platinum enhances the reducibility of the oxide. However, there are pronounced differences among all catalysts., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

The development of lignin valorization processes such as catalytic fast pyrolysis (CFP) to produce fine chemicals and fuels leads to a more sustainable future. The implementation of CFP is enabled by understanding the chemistry of lignin constituents, which, however, requires thorough mechanistic investigations by detecting reactive species. In this contribution, we investigate the CFP of the three methoxyphenol (MP) isomers over H-ZSM-5 utilizing vacuum ultraviolet synchrotron radiation and operando photoelectron photoion coincidence (PEPICO) spectroscopy. All isomers demethylate at first to yield benzenediols, from which dehydroxylation reactions proceed to produce phenol and benzene. Additional pathways to form benzene proceed over cyclopentadiene, methylcyclopentadiene, and fulvene intermediates. The detection of trace amounts of methanol in the product stream suggests a demethoxylation reaction to yield phenol. Guaiacol (2- or ortho -MP) exhibits slightly higher reactivity compared to 3-MP and 4-MP, due to the formation of the fulvenone ketene, which opens additional routes to benzene and phenol. When compared to benzenediol catalytic pyrolysis, the additional methyl group in MP leads to high conversion at lower reactor temperatures, which is mostly owed to the lower H 3 C-O vs. H-O bond energy and the possibility to demethoxylate to produce phenol.

A Viewpoint regarding our recently published Letter in this Journal (Gladich et al. J. Phys. Chem. Lett. 13, 2994-3001) criticizes some of our conclusions. While a sentence in the abstract and one in the conclusion of the Letter might seem too conclusive, in the body text we objectively discussed experimental results obtained by means of three different surface-/interface-sensitive spectroscopies. Such results were supported by theoretical calculations. The aim of our work was not to criticize past results. On the contrary, we critically discussed our data taking into account those obtained in the past.

Lignin is an abundant natural polymer obtained from lignocellulosic biomass and rich in aromatic substructures. When efficiently depolymerized, it has great potential in the production of value-added chemicals. Fast pyrolysis is a promising depolymerization method, but current studies focus mainly on small quantities of lignin. In this Review, to determine the potential for upscaling, systems used in the most relevant unit operations of fast pyrolysis of lignin are evaluated. Fluidized-bed reactors have the most potential. It would be beneficial to combine them with the following: slug injectors for feeding, hot particle filters, cyclones, and fractional condensation for product separation and recovery. Moreover, upgrading lignin pyrolysis oil would allow the necessary quality parameters for particular applications to be reached., (© 2022 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Unveiling the coke formation in zeolites is an essential prerequisite for tackling the deactivation of these catalysts in the transformations of hydrocarbons. Herein, we present the direct mapping of coke in the micropores of ZSM-5 catalysts used in methanol-to-hydrocarbons conversion by single-crystal electron diffraction analysis. The latter technique revealed a polycyclic aromatic structure along the straight channel, wherein the high-quality data permit refinement of its occupancy to about 40 %. These findings were exploited to analyze the evolution of micropore coke during the reaction. Herein, coke-associated signals, which correlate with the activity loss, indicate that the nucleation of coke commences in the intersections of sinusoidal and straight channels, while the formation of coke in the straight pores occurs in the late stages of deactivation. The findings uncover an attractive method for analyzing coke deposition in the micropore domain., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The vast structural and chemical diversity of metal-organic frameworks (MOFs) provides the exciting possibility of material's design with tailored properties for gas separation, storage and catalysis. However, after more than twenty years after first reports introducing MOFs, the discovery and control of their synthesis remains extremely challenging due to the lack of understanding of mechanisms of their nucleation and growth. Progress in deciphering crystallization pathways depends on the possibility to follow conversion of initial reagents to products at the molecular level, which is a particular challenge under solvothermal conditions. The present work introduces a detailed molecular-level mechanism of the formation of MIL-53(Al), unraveled by combining in situ time-resolved high-resolution mass-spectrometry, magic angle spinning nuclear magnetic resonance spectroscopy and X-ray diffraction. In contrast to the general belief, the crystallization of MIL-53 occurs via a solid-solid transformation mechanism, associated with the spontaneous release of monomeric aluminum. The role of DMF hydrolysis products, formate and dimethylamine, is established. Our study emphasizes the complexity of MOF crystallization chemistry, which requires case-by-case investigation using a combination of advanced in situ methods for following the induction period, the nucleation and growth across the time domain., (© 2022. The Author(s).)

Zeolites, although key materials used in industrial processes, remain poorly understood on a molecular level despite their well-defined crystal lattices. In fact, obtaining a direct spectroscopic signatures and resolving the structure of Lewis acid sites (LAS) has remained a challenge. In this work, thanks to 1D and 2D 1 H, 15 N, and 27 Al MAS NMR spectroscopy, carried out at different temperatures (from 298 down to 107 K), we were able to obtain the NMR spectroscopic signatures of LAS and Brønsted acid sites (BAS) in mordenite zeolite in the presence and the absence of adsorbed pyridine (Py). Combined with DFT modeling, this information enabled the structure of LAS to be revealed, namely (≡SiO) 3 Al sites interacting with pyridine, thus indicating that the corresponding base-free framework-associated sites are pseudo tricoordinated Al sites, namely tricoordinated Al sites interacting with an additional coordinated adjacent siloxane bridge. With this information in hand, we propose a molecular-level understanding on how the Al IV and Al VI framework and framework-associated sites evolve upon dehydration and exposure to Py into BAS and LAS, and their associated Py adducts. By measuring and analyzing the changes in quadrupolar coupling constants (C Q ) that reflect electrical charge distribution around the nuclei, we further show that the lower C Q values observed at 298 K are due to residual dynamics that makes the electric field around aluminum nuclei more symmetric. Thus, NMR spectroscopic signatures of 27 Al greatly vary with temperature; this information illustrates the importance of accounting for the temperature effect when confronting experimental and calculated C Q values of the corresponding aluminum sites in zeolites in order to obtain accurate structural assessment.

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)-induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal-oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.

Fenton chemistry, involving the reaction between Fe 2+ and hydrogen peroxide, is well-known due to its applications in the mineralization of extremely stable molecules. Different mechanisms, influenced by the reaction conditions and the solvation sphere of iron ions, influence the fate of such reactions. Despite the huge amount of effort spent investigating such processes, a complete understanding is still lacking. This work combines photoelectron spectroscopy and theoretical calculations to investigate the solvation and reactivity of Fe 2+ and Fe 3+ ions in aqueous solutions. The reaction with hydrogen peroxide, both in homogeneous Fenton reagents and at the liquid-vapor interface, illustrates that both ions are homogeneously distributed in solutions and exhibit an asymmetric octahedral coordination to water in the case of Fe 2+ . No indications of differences in the reaction mechanism between the liquid-vapor interface and the bulk of the solutions have been found, suggesting that Fe 3+ and hydroxyl radicals are the only intermediates.

The copper-zinc-alumina (CZA) catalyst is one of the most important catalysts. Nevertheless, understanding of the complex CZA structure is still limited and hampers further optimization. Critical to the production of a highly active and stable catalyst are optimal start-up procedures in hydrogen. Here, by employing operando X-ray absorption spectroscopy and X-ray diffraction, we follow how the industrial CZA precursor evolves into the working catalyst. Two major events in the activation drastically alter the copper- and zinc-containing components in the CZA catalyst and define the final working catalyst structure: the reduction of the starting copper(II) oxide, and the ripening and re-oxidation of zinc oxide upon the switch to catalytic conditions. These drastic events are also accompanied by other gradual, structural changes. Understanding what happens during these events is key to develop tailored start-up protocols that are aimed at maximal longevity and activity of the catalysts., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

We report the absolute photoionization cross section (PICS) of fulvenone and 2-carbonyl cyclohexadienone, two crucial ketene intermediates in lignin pyrolysis, combustion and organic synthesis. Both species were generated in situ by pyrolyzing salicylamide and dectected via imaging photoelectron photoion coincidence spectroscopy. In a deamination reaction, salicylamide loses ammonia yielding 2-carbonyl cyclohexadienone, a ketoketene, which further decarbonylates at higher pyrolysis temperatures to form fulvenone. We recorded the threshold photoelectron spectrum of the ketoketene and assigned the ground state (X̃ +2 A'' ← X̃ 1 A') and excited state (Ã +2 A' ← X̃ 1 A') bands with the help of Franck-Condon simulations. Adiabatic ionization energies are 8.35 ± 0.01 and 9.19 ± 0.01 eV. In a minor reaction channel, the ketoketene isomerizes to benzpropiolactone, which decomposes subsequently to benzyne by CO 2 loss. Potential energy surface and RRKM rate constant calculations agree with our experimental observations that the decarbonylation to fulvenone outcompetes the decarboxylation to benzyne by almost two orders of magnitude. The absolute PICS of fulvenone at 10.48 eV was determined to be 18.8 ± 3.8 Mb using NH 3 as a calibrant. The PICS of 2-carbonyl cyclohexadienone was found to be 21.5 ± 8.6 Mb at 9 eV. Our PICS measument will enable the quantification of reactive ketenes in lignin valorization and combustion processes using photoionization techniques and provide advanced mechanistic and kinetics insights to aid the bottom-up optimization of such processes.

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Cu diimine complexes present a noble metal free alternative to classical Ru, Re, Ir and Pt based photosensitizers in solution photochemistry, photoelectrochemical or dye-sensitized solar cells. Optimization of these dyes requires understanding of factors governing the key photochemical properties: excited state lifetime and emission quantum yield. The involvement of exciplex formation in the deactivation of the photoexcited state is a key question. We investigate the excited-state structure of [Cu(dmp) 2 ] + and [Cu(dsbtmp) 2 ] + (dmp = 2,9-dimethyl-1,10-phenanthroline, dsbtmp = 2,9-di- sec -butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) using pump-probe X-ray absorption spectroscopy (XAS) and DFT. Features of XAS that distinguish flattened tetrahedral site and 5-coordinated geometry with an additional solvent near Cu(II) center are identified. Pump-probe XAS demonstrates that for both complexes the excited state is 4-coordinated. For [Cu(dmp) 2 ] + the exciplex is 0.24 eV higher in energy than the flattened triplet state, therefore it can be involved in deactivation pathways as a non-observable state that forms slower than it decays. For [Cu(dsbtmp) 2 ] + the excited-state structure is characterized by Cu-N distances of 1.98 and 2.07 Å and minor distortions, leading to a 3 orders of magnitude longer excited-state lifetime.

The synthesis of drug substances (DS) requires the continuous effort of the pharma industry to ensure high sustainability standards. The Suzuki-Miyaura cross coupling is a fundamental C-C bond-forming reaction to produce complex DS intermediates. The present contribution points out the way in which the synthesis of DS intermediates by C-C cross coupling can be economically competitive, while minimizing waste by selecting the appropriate heterogeneous catalyst. By comparing homogeneous, immobilized heterogeneous catalysts on silica and metal-organic framework (MOF) catalysts, while considering the perspectives of academia and industry, the critical parameters for a successful industrial application of heterogeneous catalytic Suzuki-Miyaura cross coupling reactions were identified. Heterogeneous catalysts, such as MOFs, may provide a complementary platform for reducing waste and the costs of production related to such transformations.

The location of aluminum in a zeolite framework structure defines the accessibility and geometry of the catalytically active sites, but determining this location crystallographically is fraught with difficulties. Typical zeolite catalysts contain only a small amount of aluminum, and the X-ray scattering factors for silicon and aluminum are very similar. To address this problem, we have exploited the properties of resonant X-ray powder diffraction across the Al K edge, where the aluminum scattering factor changes dramatically. By combining conventional synchrotron powder diffraction data with those collected at energies near the X-ray absorption edge, aluminum is highlighted. In this way, the different distributions of aluminum in two FER-type zeolites with identical chemical compositions but different catalytic properties could be determined unambiguously. The results are consistent with previous studies, but quantitative. This approach constitutes a major advance in our fundamental understanding of the relationship between zeolite structure and catalytic activity.

Radical-mediated gas-phase reactions play an important role in the conversion of methane under non-oxidative conditions into olefins and aromatics over iron-modified silica catalysts. Herein, we use operando photoelectron photoion coincidence spectroscopy to disentangle the elusive C 2+ radical intermediates participating in the complex gas-phase reaction network. Our experiments pinpoint different C 2 -C 5 radical species that allow for a stepwise growth of the hydrocarbon chains. Propargyl radicals (H 2 C-C≡C-H) are identified as essential precursors for the formation of aromatics, which then contribute to the formation of heavier hydrocarbon products via hydrogen abstraction-acetylene addition routes (HACA mechanism). These results provide comprehensive mechanistic insights that are relevant for the development of methane valorization processes., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Zeolite mordenite (MOR) is one of the most studied zeolites for the stepwise direct conversion of methane to methanol, but it also can exist in two forms: large port and small port. Here we report that the synthesis and selection of the parent mordenite is critical for optimizing productivity, and that large-port mordenite outperforms small-port mordenite for the stepwise conversion of methane to methanol., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A promising strategy to break through the selectivity-conversion limit of direct methane conversion to achieve high yields is the protection of methanol via esterification to a more stable methyl ester. We present an aerobic methane-to-methyl-ester approach that utilizes a highly dispersed, cobalt-containing solid catalyst, along with significantly more favorable reaction conditions compared to existing homogeneously-catalyzed approaches (e.g. diluted acid, O 2 oxidant, moderate temperature and pressure). The trifluoroacetic acid medium is diluted (<25 wt %) with an inert fluorous co-solvent that can be recovered after the separation of the methyl trifluoroacetate via liquid-liquid extraction at ambient conditions. Silica-supported cobalt catalysts are highly active in this system, with competitive yields and turnovers in comparison to known aerobic transition metal-based catalytic systems., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Pd/ZnO catalysts show good activity and high selectivity to methanol during catalytic CO 2 hydrogenation. The Pd-Zn alloy phase has usually been considered as the active phase, though mechanistic studies under operando conditions have not been conducted to verify this. Here, we report a mechanistic study under realistic conditions of methanol synthesis, using in situ and operando X-ray absorption spectroscopy, X-ray powder diffraction, and time-resolved isotope labeling experiments coupled with FTIR spectroscopy and mass spectrometry. Pd-Zn alloy-based catalysts, prepared through reduction of a heterobimetallic Pd II Zn II acetate bridge complex, and which do not contain zinc oxide or any PdZn/ZnO interface, produce mostly CO. The Pd-Zn phase is associated with the formation of CO, and does not provide the active sites required to produce methanol from the direct hydrogenation of carbon dioxide. The presence of a ZnO phase, in contact with a Pd-Zn phase, is essential for efficient methanol production., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Copper-exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time-resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono-μ-oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono-μ-oxo species, while the activation energy is two times lower., (© 2021 Wiley-VCH GmbH.)

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu-exchanged zeolites have been shown promote this reaction under stepwise conditions. Over the years, several active sites have been proposed, ranging from mono-, di- to trimeric Cu II . Herein, we report the formation of well-dispersed monomeric Cu II species supported on alumina using surface organometallic chemistry and their reactivity towards the selective and stepwise conversion of methane to methanol. Extensive studies using various transition alumina supports combined with spectroscopic characterization, in particular electron paramagnetic resonance (EPR), show that the active sites are associated with specific facets, which are typically found in γ- and η-alumina phase, and that their EPR signature can be attributed to species having a tri-coordinated [(Al 2 O)CuIIO(OH)] - T-shape geometry. Overall, the selective conversion of methane to methanol, a two-electron process, involves two monomeric Cu II sites that play in concert., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)